Work acound carbon issue #55.

grahpite.o.o currently has a manually installed fix for carbon
issue #55 in place, but in case we lose that, setting MAX_CPM
to >=60 should work around the problem.

https://github.com/graphite-project/carbon/issues/55

Change-Id: I53bbe348737edc0c513bb1b34b753018d20c3d60
Reviewed-on: https://review.openstack.org/18750
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins